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Calculating Compression Ratios

September 27, 2010 | Troubleshooting & Repair

Text by Richard Peattie and Ron LaDow


[Editor] Richard Peattie and Ron LaDow have both developed Excel spreadsheets that enable you to calculate compression ratios when building up an engine.


Ron LaDow [First download Ron's spreadsheet (Excel file) for calculating compression ratios]  The spreadsheet will only calculate the compression ratio; the measurements are up to you.

Enter values in rows with italicized captions, as follows in the upper section:

  • "Cylinder Bore" is entered once in cell B-5.
  • "Arbitrary", "Net" and "Head Chamber" volumes are entered in columns E, F, G and H for each cylinder.
  • "Add or Subtract Gaskets" entered in columns C and D; each changes both cylinders on a bank as thatís whatís required.
  • In the lower section, enter:
  • "Head Chamber" and "Piston Dome" volumes plus "Deck Height" are entered in columns E, F, G, and H for each cylinder
  • Add or Subtract Gaskets" entered in columns C and D; each changes both cylinders on a bank as thatís whatís required.

In both sections, 'subtracting' a base gasket means machining the length of the cylinder, since only one is included in the tests and one must be there.

Section: Precision Matters Top End Tool

 

  1. In this section, only two measurements are required, but they have to be measured with a burette or similar.
  2. It is strongly suggested that you measure the deck height so you'll have an idea of how the ratios might be altered

Section: Individual Measurements

  1. Head chamber volumes must be measured with burette or similar
  2. Piston domes should be measured. Cast 86mm pistons have domes between 17.0 and 17.9ccs. Shasta (9.25) 86mm pistons have 18.9cc domes.
  3. Deck height is the distance between the piston deck (a flat surface) and the top of the cylinder. It has nothing to do with clay or solder on a piston dome; that's the head/piston clearance and should be measured.

Richard Peattie: [First download Richard's spreadsheet (Excel file) for calculating compression ratios]  All input values should be in the light green cells.

Ron LaDow makes a tool that replaces the head and had a fixed volume that you can measure easily and insert in the arbitrary net volume. The top part of the spreadsheet assumes you are using that tool and have measured that reference volume before starting. The spacer's volume used in the top part to measure the piston volume is then subtracted from the head volume.

One then cc's the heads and inputs that volume in the next section. You manually put in a spacer value to obtain the required compression ratio. I have allowed you to put different values under 1&2 versus 3&4. 1&2 as well as 3&4 always have to have the same value per bank.

The lower part is similar except you have to know the piston volume that is removed from the combustion chamber. Then input the deck height. That deck height has to be the deck height with the finial spacer.

The top half values were obtained from an old head that had been cut several time so the volume of the head chambers were small thus requiring the 1.25 spacer. The lower part were values supplied by the request on the 356Talk group.

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